Many types of imaging techniques are known for deriving spatial information about a target object (sometimes referred to as a specimen). However, it is often not possible to directly image a target specimen by conventional means such as brightfield microscopy. For example in conventional transmission imaging an object is irradiated by plane wave illumination. The waves scattered by the object are re-interfered by a lens to form an image. In the case of very short wave length imaging (X-rays or electrons) this technique has many known difficulties associated with aberrations and instabilities introduced by the lens which limit the resolution and interpretability of the resulting image. Typical achievable resolution is many times larger than the theoretical limit. Other types of imaging techniques are known but many of these have problems such as resolution limits, long data gathering times or the need for complex and expensive equipment.
In many instances it is possible to derive some of the properties of the specimen by measuring the way in which it scatters incident radiation. The distribution of scattered radiation at some distance from a specimen is known as a diffraction pattern and if the radiation is sufficiently coherent, it is possible to form an image of the specimen from measurement of its diffraction pattern. One technique for forming this image is named ptychography. Here, a target specimen is illuminated by a sufficiently coherent wave-front, known as the ‘probe’, whose intensity is concentrated within a localised lateral region where it interacts with the specimen. A set of diffraction patterns is then recorded by one or more detectors, with each pattern corresponding to a different relative lateral position of the specimen and probe. These positions are chosen such that an area of interest of the specimen is covered by multiple overlapping positions of the probe. An example of this technique for high resolution imaging has been disclosed in WO 2005/106531, which is herein incorporated by reference for all purposes. The technique disclosed in WO 2005/106531 is now referred to by those skilled in the art as the ptychographical iterative engine (or PIE). This involves providing incident radiation from a radiation source at a target object; detecting, via at least one detector, the intensity of radiation scattered by the target object and providing image data responsive to the detected intensity without high resolution positioning of the incident radiation or a post target object aperture relative to the target object; and using the detected intensity to produce image data for constructing an image of a region of a target object. The image data may be produced using an iterative process using a moveable softly varying probe function such as a transmittance function or illumination function.
PIE provides a powerful technique for the recovery of image data relating to an area of an object from a set of diffraction pattern measurements. Each diffraction pattern is formed by illuminating an object with a known wave front of coherent radiation with the requirement that the intensity of the wave front is concentrated within a localised lateral region where it interacts with the object. Examples of such a wave front would be that generated a short distance beyond an aperture when it is illuminated by a plane wave, or the focal spot generated by a convex lens illuminated by a plane wave. The technique is also applicable to scenarios where a target is illuminated by plane wave radiation and a post target object aperture is used to select illumination scattered by a region of the object.
In this sense a diffraction pattern is the distribution of intensity produced by an optical configuration some distance beyond the object and at a plane normal to the direction of propagation of the illumination wave front. This plane is designated as the measurement plane and measurements made at this plane are denoted ψk (u) with u being an appropriate coordinate vector. It is to be noted that when the distance between the measurement plane and a sample plane is small the diffraction pattern is known as a near-field diffraction pattern. When this distance is large the diffraction pattern is known as a far-field diffraction pattern.
Ptychography makes use of several diffraction patterns recorded at the measurement plane using a suitable recording device such as a CCD camera or the like. The lateral positions of the object and the localised illumination wave front are different for each pattern.
A limitation of PIE is the requirement that, in order to provide useful image data, characteristics of a probe function (e.g. a transmittance function associated with a post target object aperture or an illumination function associated with incident radiation) must be known or estimated. This requires time consuming set up techniques and can lead to inaccuracies if the probe function used is inaccurate.
This limitation of PIE may be addressed by a technique disclosed in WO2010/064051, which is herein incorporated by reference for all purposes. The technique described in WO2010/064051 is referred to as extended Ptychographical Iterative Engine, or ePIE. This technique begins with a rough initial estimate of the probe wave-front and a rough initial estimate of the target specimen. Each iteration of the ePIE produces updated estimates of the probe and of the specimen. The initial estimates need not be accurate; it is possible for the algorithm to produce an image given only a rough initial guess at the probe's shape. However, it is possible that the algorithm will fail to produce an accurate image. In some cases each iteration of the ePIE will produce estimates of the specimen and of the probe that are less accurate than those resulting from the previous iteration, and the algorithm is said to diverge.
It is an aim of the present invention to at least partly mitigate the above-mentioned problems.
Ptychography is applicable to imaging performed in either the reflection mode (where the illuminating beam is reflected from the target specimen) or the transmission mode (where the illuminating beam is transmitted through the target specimen.) Herein, when transmission/transmissive/transmit is used it should be understood that reflection/reflective/reflect could equally well be used.